PCI can help UEs distinguish different cells from each other by distinguishing different transmitters. In LTE, each cell is assigned a PCI upon deployment. The range of PCI is currently from 0-503. Assigning PCI to a large number of cells can create a problem because no two cells in range of a UE should be assigned the same PCI. If a UE receives communications from two cells with the same PCI, a PCI confusion or PCI collision can occur. These confusions or collisions can render the UE unable to identify the cell correctly. The number of small cells (e.g., micro, pico, or femto cells) is expected to increase in the future, thus exacerbating the problem.
A collision can occur when a UE is within range of two cells that each has the same PCI assigned and are simultaneously broadcasting to the UE. A confusion can occur when the UE is within range of two cells that each have the same PCI and the UE cannot distinguish between the two cells. Thus, to avoid or reduce the number of collisions a PCI should be unique in the area that a given cell covers, and to avoid or reduce the number of confusions a cell should not have neighbor cells with identical PCIs. Confusion can cause handover procedures from one cell to another cell to fail.